The present invention relates to a method for the removal of molybdenum from solutions containing it and also to methods for the separtion of molybdenum and tungsten in leachate solutions containing both of these mineral species.
Molybdenum is one of the major contaminants in tungsten ores. Furthermore, it is necessary to remove as much of the molybdenum as possible in order to produce high purity tungsten products. However, since molybdenum has similar properties to tungsten in terms of its oxidation potential, its solubility, and its extraction coefficient, the separation of these two mineral values by conventional means is extremely difficult.
For example, U.S. Pat. No. 2,963,343, issued Dec. 6, 1960 to Pilloton, discloses a process for the separation of molybdenum from tungsten in leachate solutions. In this process, the leachate is acidified and reacted with sodium hydrosulfide (NaSH), molybdenum being precipitated as molybdenum trisulfide. However, this process exhibits several disadvantages. These include the loss of tungsten values through the occurrence of coprecipitation with molybdenum sulfide. Additionally, difficult filtration requirements add significantly to equipment cost and operational difficulties. Lastly, this commercial method results in the generation of hydrogen sulfide, which is well known to be noxious.
Another process for the separation of molybdenum from tungsten-containing solutions is described in the article "The Recovery of Molybdenum From Spent Mandrel Acid by Solvent Extraction" by D. S. Flett, J. Mellig and E. W. West on pages 80-149 of the Proceedings of the International Solvent Extraction Conference, held Sept. 6-12, 1980 in Leige, Belgium. In this process, solvent extraction using di-2-ethylhexyl phosphoric acid (DEHPA) has been proposed utilizing the cationic nature of molybdenum at low pH levels. However, the high separation factor for molybdenum over tungsten is not achieved due to the presence of anionic molybdenum species including molybdenum-tungsten heteropolyanions.
Yet another process for the separation of molybdenum and tungsten by solvent extraction is disclosed in U.S. Pat. No. 3,969,478, issued July 13, 1976 to Zelikman et al. In this process, the molybdenum and tungsten-containing solution is acidified and reacted with hydrogen peroxide to form peroxide complexes. The separation of tungsten and molybdenum is carried out using tributylphosphate (TBP). The physical characteristics of tributylphosphate, particularly its high viscosity and low density, cause the process of Zelikman et al. to suffer from operational problems in the use of mixer/settler devices and further renders the separation of the aqueous and organic phases difficult. The difficult separation is caused by slow settling and by the formation of stable emulsions. Moreover, in using this process in a mixer/settler apparatus, it is highly desirable to maintain a ratio of organic to aqueous phase of from 1 to 1.5. However, as more particularly pointed out below, such a limitation is not at all present in the novel method disclosed herein. Furthermore, the method described in the patent to Zelikman et al. also requires a stabilizer in order to use the method with the extraction equipment. Moreover, dilution of TBP with organic solvents to counteract these problems impairs the ability of the TBP to separate the molybdenum and tungsten species.
Another process for the separation of tungsten and molybdenum by solvent extraction is disclosed in U.S. Pat. No. 4,275,039, issued June 33, 1981 to Ozensoy et al. This patent discloses a method for overcoming some of the problems associated with the process described in the above-mentioned patent to Zelikman et al. The process of Ozensoy et al. also uses reaction of hydrogen peroxide with a feed solution containing molybdenum and tungsten so as to form peroxide complexes. The process particularly employs an extracting solution comprising DEHPA, TBP and tributylphosphine oxide (TPO), and also hydrocarbon diluents of low aromatics content. However, this process still exhibits some of the problems associated with mixer/settler types of separation operation. These problems include difficult phase separation, emulsion formation, solvent loss and stringent requirement for minimum particulate content in the feed solution. Furthermore, this process is one that should be performed in a multi-stage operation to exhibit a high enrichment factor.
In U.S. Pat. No. 4,306,946, issued Dec. 22, 1981 in the name of the instant inventor, Donnan dialysis is employed as a continuous ion exchange process in the treatment of hydrometallurgical solutions. This process employs a solid ion exchange membrane as a barrier between feed and eluant solutions. However, the method is limited by the diffusion rate of ions through the solid ion exchange membrane. However, it is nonetheless a continuous method and provides many advantages over conventional batch-wise processing, which requires a complex flow scheme and consequently high equipment cost.
Another method of metal recovery is described in the article "Coupled Transport Membranes" by R. W. Baker et al. in the Journal of Membrane Science, Vol. 2, pages 213-233, 1977. The process described therein employs a single porous membrane which is impregnated with a liquid ion exchange material in the barrier. However, this process exhibits certain shortcomings, particularly membrane instability. For example, the liquid ion exchange material has a tendency to leach out from the membrane due to its finite solubility in water.
Additionally, a process for hydrometallurgical extraction is described in U.S. Pat. No. 3,957,504, issued May 18, 1976 to Ho et al. FIG. 4 of the patent to Ho et al. is particularly relevant in that it illustrates a hydrometallurgical extracting flow diagram in which a chelating fluid is recycled back to a first membrane extraction unit. However, no provision is made in the apparatus of Ho et al. for treatment of recycled chelating solutions. In particular, there is no provision for the use of an aqueous organic separator or extractor which the present inventor has found to be particularly effective in selectively isolating desired metallic species, even when mixed with solutions containing ferric or ferrous ions. Additionally, there is no provision for a peroxidation step such as is employed in the present invention. Lastly, Ho et al. are not appreciative of the particular problems associated with the separation of molybdenum and tungsten species.
The present invention is distinct from the above-described methods for several reasons, most notably in that liquid ion exchange material is not in direct contact with the aqueous feed nor impregnated in a membrane but rather is disposed in a preferably recirculating system between separate membranes in distinct fluid-tight chambers. The present invention employs indirect contact of an organic extraction solution and the aqueous leachate solution across a solid membrane barrier. It particularly enables the use of any type of organic solvent with significantly less stringent requirements on the physical properties of the organic solvent. Accordingly, the limitations on the aqueous-to-organic phase volume ratios are not even applicable to the present invention. Additionally, the present invention does not require the use of stabilizer materials to provide phase separation. Moreover, the present invention is particularly aimed at eliminating the problems of particulate contamination, emulsion formation and phase separation associated with mixer/settler methods of molybdenum/tungsten separation.